Compounds known as thickeners are commonly employed in a variety of compositions to provide thickening effects, such as in aqueous compositions, more particularly aqueous coating compositions, and still more particularly latex paint compositions.
The term "latex" has a meaning well known in the art and is used to designate an aqueous dispersion, suspension or emulsion of a water-insoluble polymer in water.
Aqueous compositions which generally employ thickeners include paints, other coating compositions such as coatings for paper, textile, upholstery fabric, rug, carpet backs, and the like, inks, adhesives, cosmetics, etc. Most of these aqueous compositions require the use of a thickening material to improve rheological properties such as to obtain a proper viscosity in order to maintain pigments, binders and other solids dispersed, suspended or emulsified therein, to obtain suitable flow properties, and to obtain good compatability.
A particularly important use of thickeners is in latex paint compositions. Cellulosics, particularly hydroxyethyl cellulose (HEC), have dominated the latex paint thickener market almost since the inception of latex paint twenty-five years ago. Hydroxyethyl cellulose is a nonionic water soluble material that thickens and stabilizes many water-based paint systems. Hydroxyethyl cellulose, however, suffers from many disadvantages. For example, hydroxyethyl cellulose is commonly sold as a solid. Consequently, it must generally be carefully pre-dispersed or dissolved in water or glycol before adding it to the paint formulation. Hydroxyethyl cellulose swells rapidly. Thus, it easily forms lumps, which are not readily dispersed during the pigment grinding stage when the HEC is added as a solid. Predispersion or dissolution of solid thickening agents of this type typically requires careful addition, long mixing times and dilution, thereby adding additional steps to the paint formulation scheme, which in turn increases the paint processing cost. It would be much more advantageous if a thickening agent could be provided in a liquid form (e.g., emulsion) thereby permitting its addition subsequent to the pigment dispersion (i.e., during the letdown stage or by post-addition). The post-addition method holds the greatest advantages for formulation flexibility, the capability of easy final viscosity adjustment, and less troublesome pigment dispersion, which are of prime importance to most paint manufacturers.
Another disadvantage of hydroxyethyl cellulose thickeners is that they are subject to bacterial and enzyme attack which results in spoilage (putrefaction) of the paint, loss of viscosity and other flow properties, and therefore require use of anti-biodegradation preservatives in paint.
The search has therefore continued for a thickener which (1) can be employed as a post-additive, directly to aqueous compositions, particularly latex paint compositions, (2) is non-biodegradable, and (3) yields a lower cost/performance ratio with respect to the overall manufacturing and rheological properties of a latex paint composition into which it is incorporated, such as in addition to thickening action, spattering resistance, hiding, gloss retention, leveling and stain removal, when compared to cellulosic materials.
The present invention was developed in response to this search.
It is therefore an object of the presently claimed invention to provide a thickener for aqueous compositions, particularly aqueous coating compositions, and more particularly latex paint compositions.
Other objects and advantages of this invention will become apparent from the following summary and description of the preferred embodiments of the present invention.